To produce a rubber tube including a reinforcing material and having a smooth outside surface, it is the conventional practice to suppress the foaming of a rubber material on the outside surface of the tube in the vulcanization of the rubber material constituting the rubber tube by using a pressurizable vulcanization device and thereby applying pressure to the peripheral surface of the tube, or by coating the periphery of the tube with a synthetic resin or lead.
In the former method of suppressing the foaming of the rubber material on the outside surface of the tube by using the vulcanization device, the foaming itself of the outside surface of the tube can be limited, but still it is impossible to achieve smoothening of the outside of the tube. Moreover, since it is necessary to seal up the inside of the vulcanization device consecutively, this method is inconveniently unsuitable for mass-production.
In the latter method of coating the periphery of the tube with lead so as to smoothen the outside surface of the tube, it is difficult to keep lead under proper care and storage, and expenditures for facilities and operations increase, and the product is likely to increase in cost.
A method comprising coating the periphery of the tube with a synthetic resin, and then vulcanizing the rubber material constituting the rubber tube was regarded as promising as a method for producing a rubber tube having a smooth outside surface. As shown in Japanese Laid-Open Patent Publication No. 290,011/1986, a technique was developed by which the periphery of a rubber tube is coated with a thermally resistant resin cover having a higher melting point than the vulcanization temperature of the rubber material, and then vulcanizing the rubber material by using, for example, microwaves.
In the conventional method of coating the periphery of a tube with a synthetic resin including the one disclosed in the above patent publication, an extrusion die for a resin is used as a resin coating device 2 as shown in FIG. 3, and a synthetic resin 6 is extruded along the periphery of a rubber material layer coated on the periphery of a mandrel to form a resin coated layer 8 continuously on the periphery of the rubber material layer 4. Since the rubber material layer 4 as coated with the synthetic resin layer is heated and thus vulcanized in the subsequent step, the synthetic resin 6 constituting the resin coated layer 8 must have a higher melting point than the vulcanization temperature of the rubber material. Otherwise, during the vulcanization of the rubber material layer 4, the resin coated layer 8 would also be melted and the peripheral surface of the rubber material layer 4 would be destroyed.
Accordingly, the synthetic resin 6 in the molten state flowing through the resin coating device 2 as the extrusion die has to be heated at a temperature above the vulcanization temperature of the rubber material. In the prior art, a heater 10 is mounted on the outside of the device 2 so as to heat the entire device. Furthermore, the rubber material layer 4 coated on the periphery of the mandrel, before being fed into the device 2, is heated by a preheater 12 so that the periphery of the rubber material layer 4 is covered closely with the resin coated layer 8.
However, in the coventional method and device 2, the device 2 is heated from outside so that the synthetic resin may remain flowable, and the rubber material layer 4 is heated with the preheater 12. Hence, during passage through the device 2, volatile components contained in the rubber material layer 4 are likely to be foamed, and thus, the peripheral surface of the rubbery material layer 4 is likely to be coated with the resin coated layer 8 while it is roughened. Accordingly, even if the surface of the rubber material layer 4 is covered closely with the resin coated layer 8, a rubber tube having a smooth peripheral surface is unlikely to be obtained.